Project tool and non-cpp file extensions

I'm using the project tool to build a plugin. I'd like to reuse a library built in my regular project structure, but I'm having a bit of trouble adding it to the Project Tool structure.
Critically, it seems like source files added to a project with "Include.Win=" must have either .h or .cpp extensions. At least, all of the files I try to add with a .cc extension aren't taking.
I should also mention that I am adding files outside of the sdk directory using "../../../../../" style paths, which I know is not recommended, but has worked well for all of my .cpp files. Could this be the problem?
Before I start extensively copying and renaming, is my assumption correct? Is there no way to force the tool to add a .cc file to my project? If so, is this documented anywhere?

I'm afraid currently the project tool does not consider .cc extensions. I'm also not aware of any option.
But I asked our development, if there might be a hidden option and otherwise it could be considered for a future version of the project tool. I'll keep you updated here.